1. Field of the Invention
The invention relates to a short arc ultra-high pressure mercury lamp. The invention relates especially to a discharge lamp for a light source which is used for a projector device, such as a liquid crystal display device or a DLP (digital light processor) using a DMD (digital micro mirror device), or the like, with a light source which is an ultra-high pressure mercury lamp in which an arc tube is filled with at least 0.15 mg/mm3 of mercury, and in which the mercury vapor pressure during operation is at least 110 atm. The invention is also directed to a process of producing such a lamp.
2. Description of the Related Art
In a display device of the projection type, there is a demand for illumination of images onto a rectangular screen in a uniform manner and with adequate color rendering. The light source is therefore a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps, and more and more often point light sources are being produced, and metal halide lamps with extremely small distances between the electrodes are being used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 150 atm, have been recently proposed. Here, the broadening of the arc is suppressed (the arc is compressed) by the increase of the mercury vapor pressure and a great increase of light intensity is the goal.
These ultra-high pressure discharge lamps are disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) and Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049).
For the above described lamp, for example, an ultra-high pressure mercury lamp is used in which there is a pair of opposed electrodes in the silica glass arc tube with a distance of at most 2 mm between them, and in which this arc tube is filled with at least 0.15 mg/mm3 of mercury and a halogen in the range of 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3. The main purpose of adding the halogen is to prevent devitrification of the arc tube. However, in this way, the so-called halogen cycle also arises.
In the above described ultra-high pressure mercury lamp (hereinafter also called only a “discharge lamp”), the phenomenon occurs that, in the course of operation, the electrodes are deformed, and that the arc discharge assumes a turbulent form. This phenomenon occurs depending on this discharge lamp or does not occur at all. When these changes of shape become greater, the discharge lamp can no longer be used.